1. Field of the Invention
This invention relates to the field of aluminum alloys, and more particularly to machinable aluminum alloys. The invention further relates to products made from such alloys, including but not limited to: screw machine stock; cold finished wire, rod and bar; extruded, cast, drawn or hot and cold rolled wire, rod and bar, and extruded, cast, drawn or hot and cold rolled forge stock.
2. Technology Review
There are several known machining alloys with 2011 and 6262 aluminum alloys (Aluminum Association designations) being among the most commonly sold. It is difficult to measure the machinability of an alloy, however. One ranking system that has been used for some time classifies machinability based on a letter scale with an "A" rating being most machinable, followed by "B", "C", "D" and "E" ratings taking into account the following characteristics:
(1) Chip Size. Smaller chip sizes are more desired because such chips simplify the machining operation and facilitate more effective heat removal from the tool-workpiece interface than larger chips. Chips must not be too small or they interfere with lubricant recirculation during the overall machining operation, such as by drilling or cutting. Long, thin chips by contrast tend to curl around themselves rather than break. Such chips, sometimes called curlings, may require manual removal from the machining area and are less effective than smaller chips at heat dissipation because larger chips tend to block the cooling lubricant. PA1 (2) Tool Wear. Lower tool wear rates are desired to save money by increasing the amount of time a tool can be used before prescribed tolerances for a given workpiece are exceeded. Lower tool wear rates further increase productivity by reducing downtime due to tool changeovers. PA1 (3) Surface Finish. Alloys exhibiting a very smooth exterior surface finish in the as-machined condition are more desired to eliminate or reduce the need for subsequent surface finishing operations, such as grinding and deburring. PA1 (4) Machining Forces. Lower machining forces are more desired to: reduce power requirements and the amount of frictional heat generated in the workpiece, tool and tool head; or increase the amount of machining or metal removal that can be accomplished with the same power requirements; and PA1 (5) Mechanical and Corrosion Properties. Mechanical characteristics such as strength, or other properties such as corrosion resistance, may be "optional" with respect to machinability. They can also be rather important depending on the intended end use for the workpiece being machined.
Although the A through E rating system is based on the five parameters discussed above, the relative importance of each parameter changes as a function of intended end use. 2011 is currently the only aluminum machining alloy that is consistently "A"-rated. This composition contains about 5-6 wt. % Cu, up to about 0.3 wt. % Zn, up to about 0.7 wt. % Fe, up to about 0.4 wt. % Si, about 0.2-0.6 wt. % Bi and about 0.2-0.6 wt. % Pb. It can be desirable to reduce the amount of lead in some products. Legislation may be requiring Pb level reductions or even elimination from certain consumer goods. A substantially lead-free substitute for 2011 or 6262 aluminum is desirable.
In the late 1930's, Alcoa patented a machining alloy consisting of 6 wt. % copper, 1 wt. % bismuth, 0.1 wt. % tin and a balance of aluminum. However, the high bismuth-to-tin ratio (10:1) preferred by U.S. Pat. No. 2,026,575, at copper levels greater than 5.5 wt. %, produce a less favorable combination of tool wear, chip size and surface finish properties.